1. Field of the Invention
The present invention relates to a beam shaping device. More particularly, the present invention relates to a beam shaping device that is used, for example, in an optical pickup apparatus to convert the laser light emitted from a semiconductor laser light source from an elliptic beam to a circular beam.
2. Description of Related Art
Optical systems for optical pickups typically use a laser diode as a light source, and the laser beam emitted from a laser diode is a divergent beam having an elliptic sectional shape. If this divergent beam as it is emitted is made to converge with an objective lens, it may illuminate only part of the circular recording region, or it may unnecessarily illuminate an area outside the recording region, either way leading to lowered recording and reproducing accuracy. To prevent this, the laser beam needs to be so shaped that it has a circular sectional shape when it strikes the recording medium.
On the other hand, in recent years, blue semiconductor lasers have come to be increasingly used as a laser light source. Because of their short wavelengths, blue semiconductor lasers require stringent accuracy in signals for recording and reproducing. Nevertheless, the outputs of currently available blue semiconductor lasers are so low that it is impossible to obtain sufficient laser power to achieve satisfactorily accurate recording and reproducing. This problem can be solved by converting the laser light from an elliptic beam to a circular beam, because this increases the efficiency with which the laser light is used. From this perspective also, techniques for beam shaping have been becoming very important.
Beam shaping is most commonly achieved by the use of a prism. However, for beam shaping to be achieved by the use of a prism, laser light needs to be collimated in advance. This requires a collimator lens, causing the following and other inconveniences. In a case where, for example, a blue laser is used, disposing a collimator lens on the light-source side of a beam shaping prism makes it impossible to move the collimator lens for the purpose of correcting the spherical aberration resulting from errors in the disk substrate.
To avoid these inconveniences, there have conventionally been proposed beam shaping devices that achieve beam shaping by the use of a lens surface. For example, Patent Publication 1 listed below proposes a beam shaping device having anamorphic surfaces on both sides; Patent Publication 2 listed below proposes a beam shaping device having cylindrical surfaces on both sides. By using these beam shaping devices, it is possible to convert a divergent beam from an elliptic beam directly to a circular beam while producing hardly any aberration.    Patent Publication 1: Japanese Patent Application Laid-Open No. H9-258099    Patent Publication 2: Japanese Patent Application Laid-Open No. 2002-208159
These conventional beam shaping devices, however, have the following disadvantages. Giving a beam shaping device anamorphic surfaces on both sides as proposed in Patent Publication 1 makes it difficult to produce the mold for forming the beam shaping device. This makes the beam shaping device unsuitable for mass production and expensive to produce. On the other hand, the beam shaping device proposed in Patent Publication 2 is of the type that enlarges the beam diameter in the minor-axis direction of the elliptic beam, and thus makes the NA (numerical aperture) of the exiting beam large. This causes the laser light to be incident at a large angle on the beam splitter for optical path integration/separation which is disposed on the downstream side of the beam shaping device, and thus makes the design of the PBS (polarizing beam splitter) film or the like difficult. Moreover, the sharply diverging beam makes it impossible to secure a sufficient distance between the beam shaping device and the collimator lens, and thus makes the arrangement of the optical components on the downstream side of the beam shaping device difficult.